1. Field of the Invention
The present invention relates to an optical network, and more particularly to a wavelength division multiplexing radio-over-fiber network that supports bidirectional wireless communication.
2. Description of the Related Art
A radio-over-fiber network transmits radio signals through optical fibers. In particular, an optical transmitter converts radio signals into optical signals and transmits the optical signals through the optical fibers. An optical receiver converts optical signals received through the optical fibers into radio signals. When wavelength division multiplexing is applied to such a radio-over-fiber network, radio signals for a plurality of remote antenna units can be transmitted through one optical fiber. Thus, such a system effectively uses the broadband width of the optical fiber.
Moreover, the wavelength division multiplexing radio-over-fiber network allows complicated electric devices dispersed in a plurality of base stations to be concentrated in a central office. In turn, a small number of optical fibers can be used because of the wavelength division multiplexing scheme.
FIG. 1 is a block diagram of a conventional wavelength division multiplexing radio-over-fiber network. The radio-over-fiber network 100 includes a base station 110, and first, second, third, and fourth remote antenna units 140-1, 140-2, 140-3, and 140-4 connected in series to the base station 110 through an optical fiber 170.
The base station 110 includes second, fourth, sixth, and eighth optical transmitters 120-2, 120-4, 120-6, and 120-8, and first, third, fifth, and seventh optical receivers 130-1, 130-3, 130-5, and 130-7. As the index i increases in an order of 1, 3, 5, 7, . . . , the ith optical receiver 130-i and the (i+1)th optical transmitter 130-(i+1) are alternately arranged and connected to one another in series. The (i+1)th optical transmitter 130-(i+1) downwardly transmits (i+1)th optical signal λi+1 having (i+1)th wavelength. The ith optical receiver 130-i receives an ith optical signal ith having an ith wavelength. Herein, the word “downstream” indicates a direction from the base station 110 to the remote antenna units 140-1, 140-2, 140-3, and 140-4, while the term “upstream” means the direction opposite to the downstream. The optical transmitters 120-2, 120-4, 120-6, and 120-8 pass the inputted optical signal therethrough. The optical receivers 130-1, 130-3, 130-5, and 130-7 receive the optical signal assigned to each optical receiver and pass the rest of the optical signals therethrough. For example, the first optical signal inputted into the base station 110 passes through the eighth, sixth, fourth, and second optical transmitters and the seventh, fifth, and third optical receivers alternately and in order, and then is received by the first optical receiver 130-1. On the other hand, the second optical signal outputted from the second optical transmitter 120-2 passes through the third, fifth, and seventh optical receivers and the fourth, sixth, and eighth optical transmitters alternately and in order, and then is downstream transmitted.
The first, second, third, and fourth remote antenna units 140-1, 140-2, 140-3, and 140-4 have structures identical to one another. The jth remote antenna unit 140-j includes a (2j −1)th optical transmitter 120-(2j−1), a (2j)th optical receiver 130-2j, a jth circulator 150-j, and a jth antenna 160-j, wherein the index j is a natural number below four.
The (2j−1)th optical transmitter 120-(2j−1) upstream transmits the (2j−1)th optical signal λ(2j-1) created by electric data signal which is inputted from the jth circulator 150-j.
The (2j)th optical receiver 130-2j converts the (2j)th optical signal λ(2j) passing through the (2j−1)th optical transmitter 120-(2j−1) into electric data signal, and then outputs the electric data signal.
The jth circulator 150-j has first, second, and third ports. The first port of the jth circulator 150-j is connected to the (2j)th optical receiver 130-2j, the second port of the jth circulator 150-j is connected to the jth antenna 160-j, and the third port is connected to the (2j−1)th optical transmitter 120-(2j−1). The jth circulator 150-j outputs electric data signal inputted into the first port to the second port, and outputs the electric data signal inputted into the second port to the third port.
The jth antenna 160-j converts radio signals received through the air into electric data signals, and then outputs the electric data signals to the jth circulator 150-j. Moreover, the jth antenna 160-j converts electric data signals inputted from the jth circulator 150-j into radio signals, and then emits the radio signals to the air.
The optical transmitters 120-1, 120-3, 120-5, and 120-7 pass the optical signals respectively inputted into the optical transmitters. The optical receivers 130-2, 130-4, 130-6, and 130-8 receive the optical signals assigned to each receiver and pass the rest of optical signals therethrough as they are. For example, the eighth optical signal λ8 passes through the first, third, fifth, and seventh optical transmitters and the second, fourth, and sixth optical receivers alternately and in order, and then is received by the eighth optical receiver 130-8. On the other hand, the seventh optical signal λ7 outputted from the seventh optical signal 120-7 passes through the sixth, fourth, and second optical receivers and the fifth, third, and first optical transmitters alternately and in order, and then is transmitted upstream.
The first to eighth optical transmitters 120-1, 120-2, . . . , and 120-8 have identical structures. The first to eighth optical receivers 130-1, 130-2, . . . , and 130-8 also have identical structures.
FIG. 2 illustrates a first optical transmitter 120-1 and a second optical receiver 130-2 included in a first remote antenna unit 140-1 shown in FIG. 1.
The first optical transmitter 120-1 includes a first housing 210, a first ferrule 215, first and second lenses 220 and 230, a first filter 225, and laser diode 235.
The first housing 210 has a cylindrical shape in which the upper and lower ends are open.
The first ferrule 215 is inserted into an upper portion of the first housing 210, and has a pair of holes into which a part of an upstream optical fiber 170 and an intermediate optical fiber 170b are inserted. The second, fourth, sixth, and eighth optical signals λ2, λ4, λ6 and λ8 are inputted into the first optical transmitter 120-1 through the upstream optical fiber 170a. The first, third, fifth, and seventh optical signals λ1, λ3, λ5, and λ7 are outputted outside the first optical transmitter 120-1. Moreover, the third, fifth, and seventh optical signals λ3, λ5, and λ7 are inputted into the first optical transmitter 120-1 through the intermediate optical fiber 170b, while the second, fourth, sixth, and eighth optical signals λ2, λ4, λ6, and λ8 are outputted outside the first optical transmitter 120-1.
The first filter 225 is disposed at an intermediate portion of the first housing 210. The first filter 225 reflects the optical signals inputted from the upstream optical fiber or the intermediate optical fiber 170a or 170b toward the intermediate optical fiber or the upstream optical fiber 170b or 170a. The first filter 225 also transmits the first optical signal inputted from the laser diode 235 toward the upstream optical fiber 170a. 
The first lens 220 is interposed between the first ferrule 215 and the first filter 225. The first lens 220 enables the first, third, fifth, and seventh optical signals to converge at an end of the upstream optical fiber 170a. 
The laser diode 235 is disposed at a lower portion of the first housing 210. The laser diode 235 converts the electric data signals inputted from the first circulator 150-1 into the first optical signals and then outputs the first optical signals.
The second lens 230 is interposed between the laser diode 235 and the first filter 225. The second lens 230 makes the first optical signals converge at an end of the upstream optical fiber 170a. 
The second optical receiver 130-2 includes a second housing 250, a second ferrule 255, third and fourth lenses 260 and 270, a second filter 265, and a photodiode 275.
The second housing 250 has a cylindrical shape in which upper and lower ends are open.
The second ferrule 255 is inserted into an upper portion of the second housing 250. The second ferrule 255 has a pair of holes into which a part of a downstream optical fiber 170c and the intermediate optical fiber 170b are inserted. The second, fourth, sixth, and eighth optical signals are inputted into the second optical receiver 130-2 through the intermediate optical fiber 170b, while the third, fifth, and seventh optical signals are outputted outside of the second optical receiver 130-2. Moreover, the third, fifth, and seventh optical signals are inputted into the second optical receiver 130-2 through the downstream optical fiber 170c, and the fourth, sixth, and eighth optical signals are outputted outside of the second optical receiver 130-2.
The second filter 265 is disposed at an intermediate portion of the second housing 250. The second filter 265 transmits only the second optical signal among the optical signals inputted from the intermediate optical fiber 170b toward the photodiode 275. The second filter 265 also reflects the remaining optical signals toward the downstream optical fiber 170c. Furthermore, the second filter 265 reflects the optical signals inputted from the downstream optical fiber 170c toward the intermediate optical fiber 170b. 
The third lens 260 is interposed between the second ferrule 255 and the second filter 265. The third lens 260 enables the optical signals reflected by the second filter 265 to converge at an end of the corresponding optical fiber.
The photodiode 275 converts the second optical signal inputted from the second filter 265 into electric data signal, and then outputs the electric data signal.
The fourth lens 270 is interposed between the photodiode 275 and the second filter 265. The fourth lens 270 enables the second optical signal to converge into a light receiving portion of the photodiode 275.
However, the conventional wavelength division multiplexing radio-over-fiber network 100 described above has a number of limitations.
For example, since each remote antenna unit has an optical transmitter and an optical receiver, the identical structural devices are repeatedly applied to the remote antenna unit. Thus, the network is complicated and the cost of realizing the network is increased.
Further, the upstream wavelength band and downstream wavelength band must be separate, thereby decreasing the efficient use of the wavelength band.